In the largest study of runners ever completed, researchers concluded that running does not increase the risk of osteoarthritis (cartilage breakdown). Runners had half the incidence of knee osteoarthritis than walkers.

"Running has been shown to thicken the cartilage in your knee," says Jason Fitzgerald, a USA Track & Field-certified coach. "The body adapts to running and gets stronger, after all. And since running doesn't involve excessive bending of the knee nor much twisting or turning, it's a very safe form of exercise for knees."

Exercise encourages bone growth

High impact exercises like running spur bone growth and strengthen the muscles around the knee, adds physical therapist Michael Silverman, P.T., M.S.P.T., coordinator of the Tisch Performance Center at the Hospital for Special Surgery.

In another widely referenced study, researchers focused on molecules that are associated with inflammation because previous research has shown inflammation in the knee may cause and worsen arthritis.

So the researchers looked for changes in the levels of several types of cells that are known to either increase or blunt the amount of inflammation inside the knee, writes Gretchen Reynolds, of The New York Times.

In almost every case, the runners’ knees showed substantially lower levels of two types of cells that can contribute to inflammation within the synovial fluid (a lubricating fluid that reduces friction inside joints), compared to their baseline levels, Reynolds added.

Running is GOOD for your knees

These findings suggest that even just a half-hour session of running changes the interior of your knee, reducing inflammation and lessening levels of a marker of arthritis, says Robert Hyldahl, a professor of exercise science at Brigham Young University (BYU) and lead author of the study.

Previous studies of synovial fluid from patients who already had osteoarthritis have found evidence that joint loading (such as running or one-legged knee extension exercises) really does trigger anti-inflammatory markers and reduce markers of cartilage turnover. That's why exercise is so highly recommended to osteoarthritis patients, to the degree they can tolerate it, according toRunner’s World.

Running may also actually protect knees later on, and safeguard against degenerative diseases like osteoarthritis.

The new finding “flies in the face of intuition,” said study co-author Matt Seeley, PhD, associate professor of exercise science at BYU “This idea that long-distance running is bad for your knees might be a myth.”

“What we now know,” added Hyldahl, “is that for young, healthy individuals, exercise creates an anti-inflammatory environment that may be beneficial in terms of long-term joint health.”